This invention relates to paralleled amplifiers, and more particularly to paralleled amplifiers interconnected by hybrids to provide channelization, where the number of amplifiers in the paralleled arrangement can be other than 2N, where N is an integer.
Multichannel spacecraft communications systems are now widely used.
Electronic amplifiers are used to boost signal strength at each end of an uplink and downlink. Power amplifiers are used to boost the power of the signal at the spacecraft before retransmission to earth. Amplifiers are basically nonlinear. If a single power amplifier were to be used to boost the signal power for all of a plurality of channels, the signals in the plural channels would become jumbled together, and, in addition to intermodulation problems attributable to the nonlinearity, the additional problem would exist that the signals could not be re-separated into separate channels without the use of frequency-sensitive filters. To avoid the need for large numbers of such filters, and also to reduce the amount of power which must be handled by a single amplifier, a number of amplifiers is used which is at least equal to the number of independent channels being transmitted. Phase-sensitive hybrid combining networks are coupled to the inputs of the amplifiers, and corresponding phase-sensitive separation hybrid networks are coupled to the outputs of the amplifiers to separate the amplified signals into the original channels. At the spacecraft, each separate channel appearing at the output of the phase-sensitive separation network may be designated, for example, for transmission over a separate antenna beam to a different portion of the Earth""s surface.
It should be noted that the paralleling of amplifiers as described above is not done for purposes of redundancy or reliability, because, if a signal amplifier among the plurality of paralleled amplifiers fails, the phase-sensitive channelization is not accomplished.
FIG. 1a is a simplified block diagram of a spacecraft communication system 10 using a prior-art paralleled amplifier. In the arrangement of FIG. 1a, communication system 10 includes a spacecraft 12 and first and second ground stations 14 and 16, respectively.
The spacecraft 12 is illustrated as including a receiving antenna 12ar and a transmitting antenna 12at. Receiving antenna 12ar forms two separate receive antenna beams, designated 12rb1 and 12rb2, directed toward ground stations 14 and 16, respectively. The signals from ground station 14 are generated at a beamformer output port 12aro1, and the signals from ground station 16 are generated at a beamformer output port 12aro2. These signals are designated A and B, for ease of notation. Thus, the A signals are transmitted from ground station 14 to receive antenna 12r by way of beam 12rb1, and the B signals are transmitted from ground station 16 by way of beam 12rb2. The A and B signals are amplified in a paralleled amplifier 20 for application to beamformer ports 12ati2 and 12ati1, respectively, of transmitting antenna 12at of spacecraft 12. Paralleled amplifier 20 has first and second input ports 20i1 and 20i2, and first and second output ports 20o1 and 20o2. The A signals applied to input port 12ati2 are transmitted to ground station 16 by way of antenna beam 12atb2, and the B signals applied to input port 12ati1 are transmitted to ground station 14 by way of beam 12tb1. Consequently, in the simple system of FIG. 1a, two separate locations on the Earth can communicate by way of pairs of antenna beams. The A and B signals may be viewed as being channels of information.
It will be clear in the arrangement of FIG. 1a that, if the A and B signals are jumbled together in paralleled amplifier 20, the signals applied from the paralleled amplifier output ports 20o1 and 20o2 to the beamformer input ports of the transmitting antenna 12at will include a mixture of both the A and B signals, and each transmit beam will transmit both A and B signals to both ground stations. This could be overcome by some method of channelization, such as by frequency division filters. However, such filters are heavy, bulky, and expensive, and therefore may not be desired. In amplifier 20 of FIG. 1a, paralleled amplifiers 1 and 2 are connected to receive the A and B signals from antenna ports 12aro1 and 12aro2 at input ports 20i1 and 20i2, respectively. The xe2x80x9cparalleledxe2x80x9d aspect of the amplifier 20 requires that there be some kind of cross-coupling of the A signals to amplifiers 1 and 2, and of the B signals to amplifiers 1 and 2. Within paralleled amplifier 20, this cross-coupling is provided by hybrids. In amplifier 20, ports 20i1 and 20i2 are coupled to the ports of a three-dB hybrid H1, well known in the art. Such hybrids are represented by crossed transmission lines in the form of the letter X, with the crossing representing the coupling between two transmission lines of the hybrid. As illustrated in FIG. 1a, the ports of hybrid H1 of FIG. 1a are designated H11, H12, H13and H14, with ports H1 and H12 being equivalent to, or contiguous with, parallel amplifier ports 20i1 and 20i2, respectively. In such three-dB hybrids, a transmission line, illustrated as a line designated T1, couples ports H11 to port H14, and another transmission line, illustrated as a line designated T2, couples ports H12 and H13. A salient characteristic of such hybrids is that, over a significant frequency band, the input signals applied to port H11 appear at ports H13 and H14 with xc2xd power, and in mutual phase quadrature, and port H12 is isolated from port H11. Similarly, signals applied to port H12 appear at ports H13 and H14 with xc2xd power, and in mutual phase quadrature.
FIG. 1b illustrates a common way to designate these characteristics. In FIG. 1b, the signal at port H11 is designated as (1,0), representing full power and a reference phase of 0xc2x0. The signal at port H12 resulting from application of (1,0) to port H11 is (0,0) representing zero power or no signal. The signals appearing at ports H13 and H14 as a result of application of signal (1,0) to port H11 are designated as (xc2xd,90) and (xc2xd,0), representing half-amplitude or half-power, with mutually quadrature phase shifts of 90xc2x0 and 0xc2x0 respectively. It should be noted that the terms xe2x80x9cinputxe2x80x9d and xe2x80x9coutputxe2x80x9d as applied to the ports of a hybrid refer only to its application, since the device is linear. A port termed an xe2x80x9cinputxe2x80x9d port in one application may well be an xe2x80x9coutputxe2x80x9d port in another application, or even in a different mode of operation of the same application.
In FIG. 1a, half-power signal A at a phase shift of 90xc2x0, corresponding to (A/2,90), appears at port H13 of hybrid H1, and half-power signal A at a phase shift of 0xc2x0 (A/2,0) appears at port H14 as a result of application of (A,0) to port H11. Similarly, half-power signal B at a phase shift of 0xc2x0, corresponding to (B/2,0), appears at port H13 of hybrid H1, and half-power signal B at a phase shift of 90xc2x0 (B/2,90) appears at port H14 as a result of application of (B,0) to port H12. Thus, amplifier 1 receives for amplification the sum of (A/2,90) and (B/2,0), while amplifier 2 receives (A/2,0) and (B/2,90). These signals, which may be designated (A/2,90)+(B/2,0) and (A/2,0)+(B/2,90), respectively, appear in amplified form at the outputs of amplifiers 1 and 2 in the usual manner. While the amplifiers may invert phase, the amplification and phase inversion are not relevant to the end result, and are ignored in the discussion. Amplified summed signal (A/2,90)+(B/2,0) appears at the output of amplifier 1 for application to an input port H21 of a 3dB hybrid H2, and amplified summed signal (A/2,0)+(B/2,90) appears at the output of amplifier 2 for application to input port H22 of hybrid H2. Hybrid H2 performs the same kind of operation as hybrid H1, and more particularly couples amplified summed signal (A/2,90)+(B/2,0) from port H21 to port H23 at half power and with a relative 90xc2x0 phase shift, and couples amplified summed signal (A/2,0)+(B/2,90) from port H22 to port H23 at half power and with a 0xc2x0 relative phase. Similarly, hybrid H2 couples amplified summed signal (A/2,90)+(B/2,0) from port H21 to port H24 with half-amplitude and relative phase of 0xc2x0, and couples amplified summed signal (A/2,0)+(B/2,90) applied to port H22 to port H24 with half-amplitude and a phase of 90xc2x0. The sum signal appearing at port H23, then, is (A/4,180)+(B/4,90)+(A/4,0)+(B/4,90), and the sum signal appearing at port H24 is (A/4,90)+(B/4,0)+(A/4,90)+(B/4,180). It will be noted that the (A/4,0) and (A/4,180) components appearing at port H23 cancel, leaving (B/4,90)+(B/4,90). When adding these two signals, which are in-phase, it should be noted that B represents RF power, whereas the usual rule of addition only applies to the amplitude which happens to be the square-root of the power. Hence the amplitude of the resulting signal is (B/4)+(B/4)=B, and the power of the resulting signal is therefore B. Hence the expression (B/4,90)+(B/4,90) is equivalent to (B, 90). The(B/4,0) and (B/4,180) components appearing at port H24 also cancel, leaving (A/4,90)+(A/4,90),which is equivalent to (A,90). Thus, the phase sensitivity of the hybrids at the input and output ports of amplifiers 1 and 2 allow each amplifier to amplify components of the input signals, but also allow separation or channelization of the signals following (downstream from) the parallel amplification. The output ports H23 and H24 of hybrid H2 are contiguous with output ports 20o1 and 20o2, respectively, of paralleled amplifier 20.
The arrangement of paralleling amplifiers as described in conjunction with FIGS. 1a and 1b has a salient advantage over simple use of two independent amplifiers. In general, the peak amplitude portions of two independent signals will not occur simultaneously, and thus the full output power capacity of each individual amplifier is used only a small portion of the time. This fact may be taken advantage of in a paralleled amplifier arrangement to allow the signals to be amplified to higher levels than in separate-single-amplifier arrangement. This is because the paralleled amplifiers can xe2x80x9capplyxe2x80x9d or xe2x80x9cusexe2x80x9d some of the capacity not used by the momentarily lower-level signals to accommodate that signal which is momentarily at a higher level. Thus, for example, if two amplifiers were available, each capable of producing an output power of one watt, separate use of those amplifiers, one for each channel, would restrict each channel to a maximum of one watt.
If those amplifiers were paralleled, however, the sum of the output powers would be limited to two watts, and it might be possible to allocate the entire two-watt output power to a single channel, if desired, at the expense of power in the other channels. Such an allocation of all power to a single channel is termed an Effective Isotropic Radiated Power (EIRP) flexibility of 100%.
It will be appreciated that an actual spacecraft-based communication system is likely to require more than two separate channels. If four channels, with signals A,B,C, and D are required, the desired channelization may be realized with a parallel amplifier arrangement such as that of FIG. 2a. In FIG. 2a, a paralleled amplifier 220 includes four amplifiers organized as two paralleled amplifiers 20, identical to those described in conjunction with FIG. 1a. To distinguish between the two paralleled amplifiers 20 of FIG. 2a, they are designated 201, and 202. The input ports of the paralleled amplifier 201, are designated 20i1,1 and 20i2,1, and the input ports of paralleled amplifiers 202 are designated 20i1,2 and 20i2,2. In each of paralleled amplifiers 201 and 202 of FIG. 2a, the input hybrid, designated H1 in FIG. 1a, is redesignated as 31, and the output hybrid, H2 in FIG. 1a, is redesignated 32. Each of the paralleled amplifiers 201 and 202 may be viewed as being equivalent to a single amplifier 1 or 2 of FIG. 1a, requiring only some sort of cross-coupling to get A and B signals to paralleled amplifier 202, and to get the C and D signals to paralleled amplifier 201. For this purpose, the A signals applied to paralleled amplifier 220 of FIG. 2a are coupled to input port 1 of a 3 dB hybrid 230, and the B input signals are applied to its input port 2. Similarly, the C input signals to paralleled amplifier 220 are applied to input port 1 of a 3 dB hybrid 232, and the D input signal is applied to its input port 2. Output port 3 of hybrid 230 is coupled to input port 20i11 of paralleled amplifier 201, and output port 4 of hybrid 232 is coupled to input port 20i22 of paralleled amplifier 202. Cross-coupling is provided by connecting, output port 4 of hybrid 230 to input port 20i12 of paralleled amplifier 202, and output port 3 of hybrid 232 is coupled to input port 20i21 of paralleled amplifier 201. With this arrangement, each of the four amplifiers contained in the combination of two paralleled amplifiers 201 and 202 amplifies signals including components of signals A, B, C, and D.
Also in FIG. 2a, output port 20o1 of paralleled amplifier 201 is coupled to port 1 of a 3 dB hybrid 234, and output port 20o2 of paralleled amplifier 202 is coupled to input port 2 of hybrid 236. Output port 20o2 of paralleled amplifier 201 is coupled to port 1 of a 3 dB hybrid 236, and output port 20o1 of paralleled amplifier 202 is coupled to input port 2 of hybrid 234. Following an analysis such as that described in conjunction with FIG. 1a, it is easy to show that the A and B signals applied to input ports 1 and 2, respectively, of hybrid 230, and the C and D signals applied to input ports 1 and 2, respectively, of hybrid 232, are separately generated, in amplified form, at the output ports of hybrids 234 and 236. More particularly, the A output is produced at port 4 of hybrid 236, the B signal is produced at output port 3 of hybrid 236, the C signal is produced at output port 4 of hybrid 234, and the D signal is produced at output port 3 of hybrid 234. It will be noted that, as in the arrangement of FIG. 1a, the output signals appear at the xe2x80x9coppositexe2x80x9d output port from the port at which they entered, and thus signal A enters at the xe2x80x9ctopxe2x80x9d of the structure, and exits at the xe2x80x9cbottom,xe2x80x9d while signal D enters at the xe2x80x9cbottomxe2x80x9d and exits at the xe2x80x9ctop.xe2x80x9d In such a four-amplifier system, there is the possibility of allocation of the entire output power of the paralleled amplifiers (four watts, for the case of one-watt amplifiers) to a single signal or channel, at the expense of the other signals; this would also correspond to an EIRP flexibility of 100%.
FIG. 2b represents the arrangement of FIG. 2a in a skeletonized form, with the amplifiers redesignated as amplifiers 1, 2, 3, and 4, and with the hybrids undesignated. The input and output ports of the paralleled amplifier arrangement of FIG. 2b are designated by the signal or channel designation, namely A, B, C, and D. The representation of FIG. 2b is easier to understand than that of FIG. 2a since it is less xe2x80x9cbusy.xe2x80x9d
It will be noted in the arrangement of FIG. 2b that an input signal applied to any one of paralleled amplifier input ports A, B, C, or D is split by the first set of hybrids, and split again by the second set of hybrids, before being applied to the input ports of the individual amplifiers. In other words, the signals applied to any one input port flow equally through all the four amplifiers.
If a larger number of channels than four must be handled, the prior art treats the four paralleled amplifiers of FIGS. 2a and 2b as a single amplifier, and uses two such paralleled amplifier arrangements, altogether including eight separate amplifiers, together with a cross-coupling arrangement for coupling all of the input signals to all of the eight amplifiers. FIG. 3 illustrates an eight-channel arrangement including the parallel combination of eight amplifiers. In the arrangement of FIG. 3, amplifiers 1, 2, 3, and 4 may be viewed as part of a four-paralleled amplifier arrangement 3201, similar to the arrangement of 220 of FIGS. 2a and 2b, and amplifiers 5, 6, 7, and 8 may be viewed as part of a four-paralleled-amplifier arrangement 3202, also similar to the arrangement 220. Taking this view, paralleled amplifiers 3201 and 3202 are cross-coupled by a set 330 of hybrids, including hybrids 330AB, 330CD, 330EF, and 330GH. Hybrid 330AB couples signals A and B from input terminals at the left of FIG. 3 to paralleled amplifier 3201, while hybrid 330cd couples signals C and D from correspondingly designated input terminals to paralleled amplifier 3201. Hybrid 330EF couples signals E and F to paralleled amplifier 3201, while hybrid 330GH couples signals G and H to paralleled amplifier 3201. Similarly, hybrid 330AB couples signals A and B to paralleled amplifier 3202, while hybrid 330cd couples C and D to paralleled amplifier 202. Hybrid 330EF couples signals E and F to paralleled amplifier 3202, while hybrid 330GH couples signals G and H to paralleled amplifier 3202. At the output end of the eight-paralleled-amplifier arrangement 320 of FIG. 4, another set 340 of hybrids, namely hybrids 340GH, 340EF, 340CD, and 340AB, provides corresponding phase-sensitive cross-coupling which results in separation of the A, B, C, D, and E signals at the correspondingly designated output terminals at the right of FIG. 3. This scheme also has an EIRP flexibility of 100%. The arrangement of FIG. 3 corresponds to that described in U.S. Pat. No. 4,618,831, issued Oct. 21, 1986 in the name of Egami et al.
It should be apparent at this stage of the description of the prior art that the technique used in the prior art to parallel amplifiers produces amplifier arrangements which have 2N amplifiers and a similar number of channels, where N is an integer. In the arrangement of FIGS. 1a and 1b, N=1, in FIGS. 2a and 2b, N=2, and in the arrangement of FIG. 3, N=3. Thus, the number of paralleled amplifiers which can be produced by this scheme include 2, 4, 8, 16, 32, . . . If more channels than eight are required, the next step up in the prior art scheme produces sixteen paralleled amplifiers (the combination of two eight-channel amplifiers, suitably cross-coupled). Such a sixteen-amplifier combination is described in the abovementioned Egami et al. patent application. In any such arrangement, the signals applied to any input port are divided so as to flow in equal amounts through each of the individual amplifiers.
It often happens that a number of channels is required which is not an integer power of two. This might occur, for example, if a ten-channel system is desired. It is clear that an eight-channel amplifier corresponding to that of FIG. 3 is insufficient, but a sixteen-channel system (the next step up), such as that described in the abovementioned Egami et al. patent, has supernumerary channels. Such supernumerary channels are undesirable, because each such channel requires its own amplifier and hybrids, which adds weight, complexity, and cost to the spacecraft, and each amplifier requires a portion of the scarce electrical resources of the spacecraft. Furthermore, travelling-wave tube amplifiers, or other types of amplifiers, may not be available for the desired power levels if particular numbers of amplifiers are used; if the system specifies ten channels with an output power of one watt per channel, the use of ten amplifiers requires one-watt amplifiers, which may be available, but the use of sixteen amplifiers requires 620-milliwatt amplifiers, which may not be available. In most applications, the disadvantages of a system with supernumerary channels preclude its use.
It should be noted that the intermodulation distortion of paralleled-amplifier arrangements relative to the prior art single-amplifier-per-channel scheme depends at least in part on the traffic or signal loading. Single frequency modulated television signals (FMTV) and single time-division multiplex (TDMA) signals tend to have lower cross-modulation and intermodulation when amplified in separate amplifiers than in paralleled amplifiers, while traffic including multiple carriers, as in mobile applications (where each beam is treated as a separate channel) provide the same performance in both the separate-amplifier and paralleled-amplifier schemes. If more amplifiers are used than the number of channels which are to be handled in a paralleled-amplifier arrangement, it is possible to divert at least some intermodulation and cross-modulation products to unused ports, thereby allowing improved performance for the paralleled-amplifier arrangements.
FIG. 4 is a simplified block diagram of a ten-channel paralleled-amplifier 420 such as might be used in the prior art in a ten-channel communication system. In FIG. 4, portion 320 is similar to the eight-channel amplifier of FIG. 3, and its separate portions are not designated individually. In the arrangement of FIG. 4, eight-paralleled amplifier 420 handles channels or signals designated A, B, C, D, E, F, G, and H. Also in FIG. 4, a two-amplifier paralleled arrangement 420 similar to 20 of FIG. 1 is provided for handling channels or signals I and J. This arrangement is functional, and has the advantage of providing but a single amplifier for each channel, but suffers from the disadvantage that the EIRP flexibility is no longer 100%. Instead, the total system power (for one-watt amplifiers) is ten watts, but signals in the two-amplifier portion 420 can have a maximum output level of two watts, which corresponds to an EIRP flexibility of 20%, and the corresponding EIRP flexibility of eight-amplifier portion 320 is 8 watts out of ten, or 80%. In other words, signals applied to input ports I and J can draw power from only two amplifiers, rather than be capable of drawing power from eight amplifiers, as with signals applied to input ports A through H.
More flexible paralleled amplifier arrangements are desired.
A paralleled amplifier arrangement, according to an aspect of the invention, includes an amplifier set including a plurality of amplifiers. Each of the amplifiers of the amplifier set includes an input port and an output port. The number of the amplifiers in the amplifier set associated with the paralleled amplifier arrangement, according to the invention, is an even number M which is not an integer power of two. The paralleled amplifier arrangement includes a plurality of input ports, equal in number to the number of input ports associated with the amplifier set. The paralleled amplifier arrangement also includes a plurality of output ports, equal in number to the plurality of output ports of the amplifier set. The paralleled amplifier arrangement includes an input combining arrangement including a number {(M/2) int(log2 M)} of input hybrids, connected in such a manner that a signal applied to any one of the input ports of the paralleled amplifier arrangement passes through the same number of the paralleled amplifiers as a signal applied to any other input port. The paralleled amplifier arrangement further includes an output combining arrangement coupled to the output ports of the plurality of amplifiers, and to the output ports of the paralleled amplifier arrangement. The output combining arrangement includes a plurality of output hybrids, equal in number to the number of input hybrids in the input combining arrangement, connected in a mirror-image manner relative to the connections of the input hybrids.